The present invention relates to a new and improved method for signal transmission, especially for the transmission of current and voltage measurement signals by a potential bridging-transmission channel with pulse modulated electromagnetic waves, for instance a waveguide. The invention furthermore relates to apparatus for the performance of such method.
Generally speaking, there is disclosed a method for signal transmission, especially for transmitting current and voltage measuring signals by means of a potential bridging-transmission channel containing pulse modulated electromagnetic waves. The input signal is compared with a follow-up signal which follows-up the input signal and a control signal corresponding to the sign of the difference between the input signal and the follow-up signal is transmitted. At the receiver side there is obtained an output signal corresponding to the input signal which is used for controlling an integrating element.
In power or high-current measurement technology efforts have been expended for quite some time to replace the potential bridging function of standard current converters by less complicated solutions. In particular, there should be circumvented the problem of high-voltage insulation between the primary side and the secondary side, leading to appreciable costs and in the case of very high voltages, exceeding 400 kV, also results in fundamental technological difficulties.
While passive solutions, for instance Faraday rotators having optical transmission channels or with microwave transmission have not yet produced satisfactory results, active solutions employing modulators at high potential and measuring value transmission by waveguides (fiber optics), simultaneously ensuring for potential separation and bridging, promise more successful results.
Since it is only possible to reliably accomplish opticl data transmission with data in binary form, the problem is especially concerned with providing suitable conversion of the analog input or measurement signal into a pulse signal which can be optically transmitted and reliably again obtaining the signal at the receiver side. For the actual data transmission there can be used, for instance, pulse-shaped frequency modulation and pulse code modulation or the like. For metering purposes there is required an amplitude accuracy, including null point stability, typically in the order of 0.1 to 0.2 percent of the rated current. Equally, the phase error tyically should not exceed about0.15 to 0.3 degrees. When working with frequencies of 50 Hz or 60 Hz this requires a real-time measurement and transmission which at most amount to about 8 .div. 17 .mu.seconds total delay.
For protective purposes the primary emphasis is placed upon extreme dynamics, i.e. a measuring range up to about 1000:1 with reduced requirements as concerns amplitude and phase accuracy, about .+-.3 .div. 5%, approximately .+-.1 to .+-.3 degrees.
Active components of the circuit may be exposed to extreme temperature fluctuations, typically for instance -25.degree. to +45.degree. or more, and extreme electromagnetic operating conditions such as lightening striking, switching operations, short-circuits on the line and so forth. Maintenance and servicing of the modulator at high potentials is extremely difficult and hardly possible without cut-off of the power line. Therefore, there is an absolute requirement of as high as possible reliability of the modulator.
The requirements for amplitude accuracy of a metering current converter can be obtained with an 11 to 12-bit PCM (pulse code modulator). At the high voltage there is used an appropriate A/D (analog-digital converter) with forwardly connected S + H (sample-and-hold circuit). The instantaneous value of the preprocessed signal, typically amplified, attenuated, filtered and so forth, is periodically stored in the sample and hold circuit, thereafter there is carried out an analog-digital conversion, usually based on the successive approximation principle. The data is then serially transmitted by means of the optical fiber guide to the receiver/demodulator. An advantage of this method is the independence of individual measurements and the thus resultant high slew-rate (follow-up speed for rapid changes of the input signal).
On the other hand, the method is associated with a number of drawbacks. It is extremely difficult to obtain the requisite phase accuracy or trueness, meaning small time delays. Of the available 8 to 17 .mu.seconds already an appreciable part is needed for the acquisition time of the sample-and-hold circuit. The analog-digital conversion and data transmission must be accomplished with extreme bit rates, in other words increased requirements are placed upon the electro-optical transmission channel.
Demodulation must be performed synchronously, and therefore standard clock-extraction at the receiver side of the system is required. In order that after turning-on a currentless line,--all of the bits are equal to null--, the measurement can be initiated without delay, i.e. settling time of the clock-extraction, it is necesary to continuously infeed a sychronization signal which, on the other hand, leads to a further increase of the bit rate to be transmitted.
A sample-and-hold circuit, in its hold mode constitutes an extremely high-ohm circuit sensitive to electromagnetic disturbances. High resolution, rapid analog-digital converters are available at the present time in hybrid circuit technology, but there is to be expected lower reliability than with monolithic circuit design. In this respect reference should be made to Military Standardization Handbook MIL-HDBK-217B.